Other kinds of harvesting machines, such as beet or potato harvesters, dig roots and tubers from the soil. Balers collect hay that has been previously cut using a mower or reaper; and flax harvesters pull flax from the ground. In some countries wheat crops are cut and left to day on a field before being collected by a combine harvester equipped with a draper header.
In a combine harvester, cut ears of crop are conveyed inside the machine, where grains are separated from the remainder of the biological matter therewith in a threshing mechanism.
The resulting, cleaned grains are conveyed to a clean grain tank within the combine harvester while short pieces of straw, chaff, tailings, husks, dust, etc. (herein “secondary products”) are returned to the field.
Such terms are used principally throughout this specification for convenience and it should be understood that these terms are not intended to be limiting. Thus “grain” refers to that part of the crop which is threshed and separated from the discardable part of the crop material which is referred to as “straw”. Incompletely threshed ears are known as “tailings”. Also, the terms “forward”, “rearward”, “upper”, “lower”, “left”, “right”, etc. when used in connection with the combine harvester and/or components thereof, are determined with reference to the combine harvester in its normal operational condition and may refer either to the direction of forward operative travel of the combine harvester or to the direction of normal material flow through components thereof. These terms should not be construed as limiting.
Not all the ears of corn are completely threshed in the threshing mechanisms so a combine harvester typically includes both a forward grain path (i.e. the normal path between its cutter bar and the clean grain tank); and a return path, that is a physical feedback path via which incompletely threshed ears are recycled to the start of the threshing process for re-threshing.
In recent years it has become increasingly important to be able to log the yield of a crop-producing field, with high accuracy.
One of the many reasons for which such logging is desirable is that a farmer may wish to know which parts of a field produce a high crop yield; and which parts a low yield. The farmer can then use a yield map of the field, made by storing in digital form yield data logged at the time of harvesting the field, to control e.g. a computer controlled fertiliser spreader attached to a tractor in order to improve the fertility of the low yield portions. The yield map may also be used e.g. for identifying field portions where intensified weed control is required.
Since agrochemicals usually constitute an important cost factor it is important for economic reasons that the farmer sprays no more of a field than is necessary.
Correct dosing of different parts of a field is also important for environmental reasons. For example, some agrochemicals actually reduce soil fertility if they are applied in too high a concentration; and of course farmers perpetually seek to minimise rainwater run-off of agrochemicals into drainage channels and thence into the local environment.
A combine harvester is in principle ideal for logging field yield data since it travels about the field while it harvests crop, thereby facilitating contemporaneous logging of site-specific yield data.
However various inherent features of combine harvester designs render the yield data logged during harvesting less accurate than they might otherwise be.
One significant factor is that it is neither possible nor desirable to log crop yields at the cutter bar of the combine harvester. This is partly because the width over which the cutter bar cuts crop makes it impossible to log all of the crop without seriously impeding the flow of crop and thereby reducing the work rate of the harvesting machine. Also, of course, the locality of the cutter bar is a harsh environment. Consequently the output signals of cutter bar-mounted transducers are likely to include considerable noise.
Furthermore it is of course much more valuable to know the yield of grains, as opposed to the yield of grains plus secondary harvesting products commonly referred to as material other than grain (“MOG” herein). The former kind of yield data is not obtainable at the cutter bar because at that stage no separation of grains from the MOG occurs.
For these and other reasons the harvested crop yield is measured in a combine harvester using a grain mass flow rate sensor located at the end of the forward grain path, beyond the branch that defines the return path.
Measuring the grain yield at such a location in a combine harvester is less accurate than it might be, for the following reasons:    1. There is a time delay of Δt seconds, wherein typically 9≦Δt≦20s, between cutting of crop at the cutter bar and the cleaned grain impacting the mass flow sensor;    2. The dynamics of the forward and return grain paths introduce noise that must be eliminated from any model used for estimating the true crop yield from a grain mass flow sensor output.
There are presently two main approaches to the calculation of yield estimates from the output signal of the grain mass flow sensor.
The so-called “Classical Approach” to yield mapping systems derives the yield from the formula:
                                          y            ^                    ⁡                      (            t            )                          =                                                                              m                  .                                ^                            ⁡                              (                t                )                                                                                      v                  ^                                ⁡                                  (                  t                  )                                            *                                                w                  ^                                ⁡                                  (                  t                  )                                                              =                                                                      m                  .                                ^                            ⁡                              (                t                )                                                                    s                ^                            ⁡                              (                t                )                                                                        (        1        )            
Wherein ŷ(t) is the derived yield ratio (kg/m2), {dot over ({circumflex over (m)}out(t) is the measured mass flowrate (kg/s) derived from the output signal from the mass flow rate sensor; {circumflex over (v)}(t) is the forward speed (m/s) of the combine harvester, determined using e.g. a radar Doppler sensor or equivalent device; and ŵ(t) or working width is the effective width (m) of the cutter bar of the harvester, i.e. the width of the cutter bar section actually engaging and cutting the crop. The speed and width factors may be combined into a single estimated surface rate ŝ(t) (m2/s).
The yield result for each instant cannot be related directly to the location where the combine harvester was at the time of the yield measurement. As noted hereinabove, there is a substantial time delay between the moment the grain stalks are cut and the moment the threshed grain passes through the mass flow rate sensor. This delay depends on the type of harvester and on the location of the sensor.
Accordingly the mass flow rate signal at time t(x) is divided by the speed and cutter bar working width values at the time t(x−Δt). The resulting yield estimate is allocated to the location of the combine at time t(x−Δt), when the collected data are stored in a memory device or used to generate a field yield map.
The classical system does not take account of the filtering or smoothening action resulting from the threshing process. A step change in the rate of ingestion of crop into the inlet of the header (e.g. when entering a field) does not result in a sharp step function at the mass flow rate sensor. There is some “smearing” effect.
It has therefore previously been proposed to model the estimated grain yield ŷ1(t) at time instant t by a technique (referred to herein as “Inverse Dynamics” filtering) involving filtering the grain mass flow sensor output signal with the inverse of a function P(s) that models the dynamics of the forward and return grain paths.
Such a technique has been found to model well the changes in grain feed rate that derive from changes in the field conditions encountered by the combine harvester. However the available output signal contains much high frequency noise, so it is also necessary to apply a low pass filter to the signal. Depending on the choice of cut-off frequency of the low pass filter, much valuable data may be lost during the low pass filtering, with the result that, overall, inverse dynamics filtering tends to underestimate the true grain yield under certain circumstances. Furthermore, the function P(s) itself presents some low-pass characteristics, such that the inverse function tends to exaggerate the influence of random variations of the output signal. Hence, the use of “Inverse Dynamics” does not necessarily provide a true picture of the actual field rates.